1. Field of the Invention
The present invention relates to a rotary shaft for transmitting power and more particularly it relates to a power transmission shaft represented by a propeller shaft or drive shaft used as a power transmission shaft for automobiles.
2. Prior Art
The propeller shaft used as the power transmission shaft of an automobile is a propeller shaft for transmitting power from the variable speed gear device to the speed reduction gear device and is connected to them through constant velocity joints installed on the opposite ends thereof, the construction being such as to be capable of accommodating changes in length and angle caused by changes in the relative position between the variable speed gear device and the speed reduction gear device.
As the joints and the intermediate shaft disposed between the joints, which constitute the propeller shaft, it has been common practice to use steel articles. Further, from the viewpoint of bending rigidity, a longer shaft is constructed such that it is split into three or four portions and the intermediate region is supported by a center bearing support. Therefore, it has been required to improve such construction from the viewpoint of weight, cost, etc.
Thus, recently, as exemplified in FIG. 10, it has been proposed to use a hollow shaft made of fiber reinforced plastic (hereinafter referred to as FRP) of high bending rigidity (see Japanese Patent Kokai Hei 3-249429). This change of material from steel to FRP makes it possible not only to achieve weight reduction but to use a longer shaft while making splitting unnecessary and dispensing with the intermediate support bearing, in which respect it becomes possible to reduce weight and cost.
In this connection, in order to secure the strength of the joined portions to realize torque transmission when joining an intermediate shaft of FRP to metal parts at the shaft ends, it has been common practice to make the cross-sectional shape of the shaft ends polygonal, to roughen the contact surfaces as by knurling in the portion where the hollow shaft ends overlap, to crimp the hollow shaft of FRP, or to force a metal part into the core of the hollow shaft, thereby achieving the joining. Further, there are various other means contrived, including applying an adhesive for joining to the contact interface between the FRP hollow shaft end and a metal part, and making use of processing, such as surface roughening, crimping, or press-fitting, combined with an adhesive, so as to retain the joining strength.
With these methods, however, cause problems in an aspect of formation; the processing of the shaft ends becomes difficult, the outer diameter has to be increased in order to secure the strength of the joined portion or axial slip-off preventive measures have to be additionally taken in order to secure reliability. Further, the methods which involve crimping an FRP hollow shaft or press-fitting a metal part into the core of the hollow shaft, entail a decrease in binding force during press fitting due to creep or stress relaxation in the FRP; thus, circumferential slip or axial slip-off sometimes occur, having serious shortcomings including a lack of long-term reliability in the product functions.
When attention is paid to the joined portion, it is seen that it is only through the area of contact between the FRP and the metal part that torque transmission either using the friction force utilizing as the drag the binding force exerted during press-fitting or using the chemical or physical adhesive force of an adhesive applied to the contact interface is effected. In this case, trying to cope with an excessive torque which is impulsively produced, one increases the amount of press fit so as to maximize the area of the contact interface or increases the amount of elastic deformation of the FRP caused by press fitting. During manufacture and processing, however, cracks are produced in the FRP or creep or stress relaxation during use cannot be avoided, thus producing problems in joining.
On the other hand, in the case where a hollow shaft made of FRP is used as an intermediate shaft in a propeller shaft which is a power transmission shaft in order to provide for lightening, low fuel consumption, cost reduction, antivibration, and noise reduction, there is a problem which has to be solved that the outer diameter of the hollow shaft has to be decreased in consideration of the limited space in an automobile.
The present invention relates to providing a power transmission shaft constructed in such a manner as to ensure realization of appropriate torque transmission under normal load conditions while securing sufficient joining strength against the impulsively exerted excessive torque, and to retain the reliability in the joined portions during long-term use. The present invention provides for a power transmission shaft constructed by the winding of a membrane, film, foil or thin sheet in layers. The power transmission shaft comprises a longitudinal middle portion composed of FRP layers, end portions composed of metal layers, and a transitional portion disposed between the middle portion and each end portion and composed of a composite layer of FRP layers and metal layers. The joining, as by welding, pinning, press-fitting, or friction welding, of metal ports such as joint elements, is effected at the ends or the ends and transitional portions. The ends or the ends and transitional portion secure the strength necessary for joining to joints or the like and enable the joining which can be retained for a long time.
An object of the invention is to provide a power transmission shaft intended to meet the above requirement for improvements, constructed in such a manner as to enable the joining of metal parts to the ends of an FRP hollow shaft to be effected by welding, pinning, press fitting or the like, to ensure realization of appropriate torque transmission under normal load conditions while securing a sufficient joining strength against impulsively exerted excessive torque, and to retain the reliability in the joined portions during long-term use.
As technical means for achieving the object, the present invention provides a power transmission shaft, wherein the longitudinal middle portion is constituted by an FRP laminated structure made of FRP wound into a pipe form, the shaft ends are constituted by a metal laminated structure made of a winding of metal membrane (film), foil or thin sheet, and the transitional portion between the middle portion and the shaft end is constituted by a composite laminated structure in the form of a combination of the FRP laminated structure and metal laminated structure. The shaft ends or the shaft ends and transitional portion secure the strength necessary for joining to joints or the like and enable the joining which can be retained for a long time to be made.
In other words, in the portions adjacent the shaft ends, the shaft comprises, successively from the middle portion toward the shaft ends, a laminated structure of FRP alone, a laminated structure having a combination of FRP and a metal film, and a laminated structure of metal film alone. In the transitional portion having a combination of FRP and metal films, the metal film is bonded to the FRP films as it is sandwiched therebetween and such laminated structures are united in layers, whereby the area of joining is remarkably enlarged. In respect of any of the circumferential and axial components of a force applied to the power transmission shaft, the force can be transmitted with a sufficient enduring strength even if a high shear stress is produced.
Further, since the part associated with the joining at the shaft ends is not a single FRP body, it is possible to effect such a reliable perfect joining method as welding or friction pressure welding for joining a metal laminate and a metal part. For example, in spite of the fact that the FRP uses a plastic material as a matrix even in the transitional portion having a combination of FRP and metal films, its lamination-bonding to the metal films greatly improve creep and stress relaxation characteristics; therefore, even if such joining method as press fitting is employed, such drawbacks as circumferential slippage and axial slip-off will not occur at all and hence the reliability of the joined portion can be retained for a long time.
Further, by arranging the angle of orientation of fibers of the FRP constituting the laminate such that plies of 0xc2x0, 90xc2x0 and xc2x145xc2x0 with respect to the axis of the hollow shaft are combined, it is possible to adjust the bending rigidity and torsional rigidity and prevent radial deformation (buckling). At this time, as regards the number of plies, it is also possible to provide a suitable combination to constitute a laminated structure according to the rpm and torque associated with the power transmission shaft (for example, propeller shaft).
In order to increase the critical rpm of the power transmission shaft (for example, propeller shaft) it is desirable that the fibers constituting the lamination be material having a low density and a high modulus of elasticity. Examples of such fibers are PAN type and pitch type carbon fibers, silicon carbide fiber, alumina fiber, boron fiber, glass fiber, para-type aramid (Kevlar) fiber, metal (steel, aluminum alloy, titanium alloy, copper, tungsten) fibers.
For application to (the intermediate shaft in) the propeller shaft, the modulus of elasticity in tension of a fiber is 1,000 kgf/mm2 (9.8 Gpa) or above, preferably 2,000 kgf/mm2 (19.6 Gpa) or above. If it is less than 1, 000 kgf/mm2 (9.8 Gpa), the critical rpm of the propeller shaft cannot be increased, whatever fiber orientation angle of FRP may be arranged.
The strength of a fiber is 100 kgf/mm2 (980 Mpa) or above, preferably 200 kgf/mm2 (1960 Mpa) or above. If it is less than 100 kgf/mm2 (980 Mpa), the structure is insufficient in strength against the torque acting on the propeller shaft, whatever fiber orientation angle of FRP may be arranged.
Two or more of these fibers may be combined for use. Fibers which are high in specific strength and specific modulus of elasticity are effective for weight reduction and suitable for use for the propeller shaft. That is, PAN type carbon fiber is suitable from the viewpoint of specific strength and pitch type carbon fiber is suitable from the viewpoint of specific modulus of elasticity. From the viewpoint of cost reduction, a combination of these carbon fibers or a hybrid combination of these carbon fibers and glass fiber may be used.
These fibers may be in tow form or prepreg form. In the case of tow form, it is formed into a thin-walled large diameter article by the filament winding method while it is immersed in an uncured matrix resin. In the case of prepreg form, it is formed into a thin-walled large diameter article by the pipe rolling method. For the formation of a laminate in which the fiber orientation angle of FRP is arranged using a combination of plies of 0xc2x0, 90xc2x0 and xc2x145xc2x0 with respect to the axis of the hollow shaft, the pipe rolling method using prepreg is suitable. With the filament winding method, it is difficult to provide a fiber orientation angle of 0xc2x0. The prepreg used in the pipe rolling method is a half-cured sheet of fibers impregnated with thermosetting resin, enabling the disposition of the threads to be maintained in a given direction, a laminating process to be performed in a stabilized manner, and winding to be effected with an optional fiber orientation angle. The fiber sheet to be used herein may be a cloth with threads interlaced in advance at right angles in addition to a given direction.
Thermosetting resins for impregnation as a matrix are not particularly limited. Generally, among usable resins are thermosetting property exhibiting epoxy resin, phenolic resin, unsaturated polyester resin, vinyl ester resin, urethane resin, alkyd resin, xylene resin, melamine resin, silicone resin, and polyimide resin. From the viewpoint of strength, epoxy resin is suitable. When an epoxy resin is used as a matrix, its heat resistance should be not less than 60xc2x0 C., more preferably not less than 80xc2x0 C. after the curing of epoxy is used. The atmospheric temperature for the propeller shaft used as a power transmission shaft for an automobile is about 60xc2x0 C.; therefore, if the heat resistance after the curing of epoxy is less than 60xc2x0 C., serious problems such as rupture could occur, and it cannot be used as a matrix.
It is possible to use a modified epoxy resin having impact strength imparted thereto by adding rubber particles in the epoxy resin to form an island structure, and another modified epoxy resin whose principal and side chains structurally modified. Further, it is possible to use an epoxy resin having such a filler as electrically conductive carbon black and metal powder dispersed therein to provide electric conductivity. When this resin is used, electric welding, such as spot welding, becomes possible. Further, the interface strength between the matrix and fibers can be improved by surface-activating the surfaces of the fibers to be impregnated by ozonic oxidation treatment or ultraviolet radiation, by improving affinity by wet treatment using a silane coupling agent or titanium coupling agent, or by forming a high reactive functional group site on the fiber surfaces so as to provide firm adhesion having chemical boding after curing with a thermosetting matrix resin.
In the case of forming a laminated structure by using the pipe rolling method, a metal membrane (film), foil or thin sheet to be used on the shaft ends can be wound on a pipe or the like, and there is no particular limitation on the metal so long as it can be put to machining, such as drilling, welding or friction welding. However, preferable examples are iron, aluminum, copper, titanium, and tungsten. Alloys of any of these metals may also be used. Further, the surface of the metal membrane (film), foil or thin sheet used on the shaft ends maybe subjected for surface activation to ozonic oxidation or ultraviolet radiation, or wet-treated with silane coupling agent or titanium coupling agent so as to improve affinity, or may have a highly reactive functional group site formed on the metal surface, providing firm adhesion having a chemical bond with the thermosetting matrix resin after curing, thereby increasing the strength of the interface between the matrix and the metal surface. The surface of the metal membrane (film), foil or thin sheet used on the shaft ends may, in combination with the surface treatment or singly, be subjected to a surface roughening treatment. Examples of the surface roughening treatment mentioned herein are sand blasting, such physical roughening treatments as drawing, pressing and rolling, and a chemical corrosion treatment using chemicals such as nitric acid and chloric acid.
When a metal membrane, foil or thin sheet is used to form a laminated structure on the shaft ends by the pipe rolling method, an adhesive may be used in clearances between the layers of metal membrane (film), foil or thin sheet to bond them together. The adhesive to be used may be any of all that can be commonly industrially used. However, a film-like hot melt type heat adhesive tape is suitable from winding and post-cure film thickness control aspects. Among this type of heat adhesive tapes are epoxy type, nitrile phenol type and nylon type, but the use is not particularly limited to these types. Further, a solution type adhesive is preferably a structural adhesive consisting of an epoxy type adhesive containing aluminum power or iron oxide powder and enabling resistance welding represented by spot welding. The viscosity of the epoxy type adhesive at this time is preferably 50-10,000 poise (5-1,000 Paxc2x7s). With less than 50 poise (5 Paxc2x7s), the adhesive would sometimes flow out when the metal film is being wound on a mandrel, while with 10,000 poise (1,000 Paxc2x7s) or above, drawbacks occur in processing, including a drawback that uniform application of the adhesive on the metal film is difficult.
When a film-like heat adhesive tape is used, the operation comprises drilling the tape surface, winding the metal membrane (film), foil or thin sheet together with the adhesive tape placed thereon, securing an electric current passage by allowing the layers of metal membrane (film), foil or thin sheet to come in direct contact with each other at the tape openings, forming nuggets by resistance welding represented by spot welding, while achieving the joining by utilizing the adhesion curing due to the welding heat.